More specifically, the method of coding/decoding and the system, which are the subjects of the invention, are more especially adapted to the implementation of high or very high bit rate radiofrequency transmission systems usable within the framework of mobile telephony or, as appropriate, in the field of radiofrequency links between electronic apparatus, in any environment, especially harsh.
In the aforementioned fields of application, the transmission of digital data with a high degree of reliability and security comes up against a major obstacle, that of the transmission of these data by way of a variable transmission channel whose characteristics are not known a priori. The digital data transmitted are subdivided into symbols, consisting of strings of bits of these data, each symbol allowing the modulation of a carrier radio wave transmitted over the channel.
The very strong demand for reliable high bit rate radiofrequency transmission processes has brought about the initiation and execution of numerous research projects relating to the definition and implementation of future-generation TDMA (Time Division Multiple Access) mobile radio communication systems.
Radiofrequency transmission channels are known by the fact that they are both frequency selective and time varying. The temporal variation is consequent upon the mobility or the speed of the user or users. Their frequency selectivity results from the conditions of propagation of the radiofrequency signals via multiple paths and the destructive superposition of the signals received, arising from propagations over these various paths. The phenomenon of frequency selectivity brings about a phenomenon of intersymbol interference, prejudicial to the quality of transmission and detection of these symbols upon reception thereof. The phenomenon of intersymbol interference and the complexity of the receivers are substantially heightened with the transmission bit rate.
These specific characteristics of the above-cited radiofrequency transmission channels have always led to the implementation of particularly subtle tailored radiofrequency interfacing systems, all the more so when high bit rate transmission with high spectral efficiency is sought.
Nevertheless, the aforesaid frequency selectivity and temporal variation, regarded a priori as major obstacles, of radiofrequency transmission channels have however been the subject hitherto of investigations, by way of the concept of diversity, as will be explained hereinbelow.
Among the aforesaid projects and developments, the process of coded modulations with bitwise interleavings, also referred to as Bit Interleaved Coded Modulations or BICM, have been known for around ten years.
In general, as described hereinbelow with reference to FIG. 1a, these processes consist in applying to an original digital stream an outer code, Co, followed by an interleaving, π, on the bits of a block of specified length in terms of number of bits, then a GRAY modulation. For a more detailed description of these processes, reference may usefully be made to the article entitled “8-PSK Trellis Codes for a Rayleigh Channel”, published by Ephraim ZEHAVI, IEEE Transactions on Communications, vol. 40, No. 5, 1992.
The processes of this type have recently formed the subject of new theoretical or practical studies. Among the aforesaid studies, mention may be made of those published by G. CAIRE, G. TARICCO and E. BIGLIERI entitled “Bit Interleaved Coded Modulation”, IEEE Trans. Inform. Theory, vol. 44, No. 3, pp. 927–946, May 1998 and by X. LI and J. A. RITCEY entitled “Trellis-Coded Modulation with Bit-Interleaving and Iterative Decoding”, IEEE ISAC, vol. 17, No. 4, pp. 715–725, April 1992.
More recently, the aforesaid studies have been generalized to a system with multiple antennas in transmission and reception.
Such a process, which is more elaborate, corresponds to the illustrative scheme as represented in FIG. 1b in which the demultiplexing on a given number of pathways, after the interleaving process, makes it possible to implement the multiple transmission process, as is represented in the aforesaid figure.
Such a process is essentially implemented for a Rayleigh channel, that is to say for a transmission channel for which the memory effect is almost absent, i.e. in the absence of any intersymbol interference phenomenon.
Such a process cannot therefore easily be implemented in a real environment in which the intersymbol interference phenomenon is always present, in particular within the framework of high or very high bit rate transmissions, the conditions of high or very high bit rate transmission having the effect of greatly increasing the aforesaid intersymbol interference phenomenon, by reason of the aforesaid memory effect. For a more detailed description of the abovementioned process, reference may usefully be made to the article entitled “Bit-Interleaved Coded Modulations for Multiple-Input Multiple Output Channels” published by J. J. BOUTROS, F. BOIXADERA, C. LAMY, IEEE 6th Int. Sum. on Spread Spectrum Tech. & Appli. NSIT New Jersey, USA, Sep. 6-6, 2000, and to the article entitled “Turbo Coded Modulation for Systems with Transmit and Receive Antenna Diversity” published by Andrej STEFANOV, Tolga M. DUMAN, Telecommunications Research Center, Electrical Engineering Department, Arizona State University, Tempe, Ariz. 85287-7206, Global Telecommunications Conference—Globecom 99.
The technique implemented according to the aforesaid process thus has the major drawback of not supporting the frequency selectivity of the elementary transmission channels, from transmission antenna to reception antenna, constituting the global transmission channel. Stated otherwise, the aforesaid technique appears unsuited to the implementation of a correction of the intersymbol interference phenomenon.
An advantageous coding has recently been proposed which implements radiofrequency interfaces, making it possible to guarantee high spectral efficiency, in the publication: “Space-Time Bit-Interleaved Coded Modulation over Frequency Selective Fading Channels with Iterative Decoding”, by A. M. TONELLO, GLOBECOM 2000, vol. 3, pages 1616–1620, San Francisco, USA, November 2000. However, the coding/decoding principles stated in this publication, although promising, make provision, on reception, for operations of multilayer detection and intersymbol interference decoding, on the one hand, and of outer decoding, on the other hand, in a disjoint and iterative manner, while making use of optimal algorithms. A decoding system based on such a principle is of great complexity and difficult to implement, in practice.